Arc lamp controller



May 14, 1968 w GREEN ET AL 3,383,557

ARC LAMP CONTROLLER Filed May 2, 1966 2 Sheets-Sheet l m IIIMIM a A w w,

IA/ZS INVENTORS BENJAM/IV W. GREEN EDWARD W. STARK ATTORNEY y 1968 B. w. GREEN ET AL 3,383,557

ARC LAMP CONTROLLER Filed May 2, 1966 2 Sheets-Sheet 2 iIIlILE Q n N N N H k \r N W l? R Q 2 Q N N 2 I {J xiv Win50 N I i Q E a: M $42 Kg w A wkfl KW 3 N N 3 m m VEVV N N N X O Q l L) f INVENTORS BE/VJAMl/V W GREEN zDIV/JRD W. STARK B A TOR/V5) Unite ABSTRACT OF THE DISCLUSURE A circuit for stabilizing the light output of an arc lamp includes a clock pulse source that energizes the lamp through a switching circuit. The electrical input to the lamp is monitored and used to control the switching circuit so as to block the flow of selected portions of each clock pulse as determined by the monitoring signal.

The invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of the Army.

This invention relates to controllers for arc lamps and more specifically to controllers for stabilizing the output of an arc lamp having a fixed electrode spacing.

Arc lamps are usually regulated by adjusting the spacing between electrodes. Some types of arc lamps such as the recently developed mercury and xenon short are lamps, however, have electrodes that are fixed in position. Input power must be adjusted to regulate the light output and to compensate for electrode burn-back.

Devices for maintaining the light output from such lamps at a desired level have been devised in which a photosensitive device is used to sample the light output. The signal from the photosensitive device is then fed back to adjust the light output of the lamp.

In some instances, however, it is impractical to place a photocell in the light beam. Furthermore, the sensitivity of such cells changes with time so that the reliability of the circuit is impaired.

It is an object of the present invention to provide a regulating system for shart-arc lamps that is reliable and stable.

It is another object of the present invention to provide a regulating system for short-arc lamps that responds rapidly to changes in lamp loading.

These and other objects are achieved in accordance with the principles of the present invention by providing a source of pulsating power to energize the lamp and by adjusting the duty-cycle of the pulsating power in response to the variations in the lamp loading.

The principles and operation of the present invention may be understood by referring to the following description and the accompanying drawings:

FIGURE 1 is a circuit diagram of one embodiment of the invention; and

FIGURE 2 is a circuit diagram of a second embodiment of the invention.

FIGURE 1 depicts a circuit that may be used with a commercial xenon lamp employing bridge wire starting and in which the lamp intensity is adjusted in response to changes in lamp current.

Lamps employing bridge wire starting are useful in situations in which the lamp is to be used only once. This type of lamp contains a filament which is heated to initiate ionization of the gas in the lamp. The filament is destroyed in the operation. Circuits using this type of lamp are herein illustrated so as to simplify the drawings and description. However, it is to be understood that other types of lamp using auxiliary starting circuits for repeated operation may be used where desired. Lamps and auxili- States Patent ary starting circuits are known in the art, Several types of lamps and starting circuits are described for instance in Catalog 1-64 copyrighted by Pek Labs Incorporated of Sunnyvale, Calif. in 1964 and entitled Mercury and Xenon Short-Arc Lamps.

Referring again to FIGURE 1, a Xenon lamp 11 is energized from a power source 13 represented in the figure as a battery. Energy from the battery 13 is applied to the circuit through a negative bus 14 and a switch 15. The positive terminal of the source 13 is connected to the circuit through a positive bus 16. A source of clock pulses 17 is used to provide negative-going clock pulses. The clock source may conveniently comprise a conventional unijunction oscillator and typically may be set to provide output pulses at a rate of 3,000 pulses per second.

The pulses from the clock source 17 are applied to an input diode 19 and a first switching transistor 21. The collector of the transistor 21 is coupled to the negative bus through a resistor 23. The emitter of the transistor 21 is coupled to the positive bus 16 through a diode 25 and a resistor 27.

A speed-up network 29 containing a capacitor 31 and a shunt resistor 33 is connected to the base of the first switching transistor 21 and to the collector of a second switching transistor 35. The transistor 35 is coupled to the negative bus through a resistor 37. The base of the transistor 35 is coupled to the collector of the transistor 21 through a coupling capacitor 39 and a coupling Zener diode 41. The output of the transistor 35 is taken from the collector of this transistor and is applied to a conventional Darlington pair 43 containing the transistors 45 and 47 through a base clearing circuit containing the resistor 49, the capacitor 51, and the diode 53.

A load resistor 55 couples the xenon lamp 11 to the negative bus. A control transistor 57 is coupled to the negative bus through a resistor 59 and to the base of the transistor 35 through a line 61. The base of the transistor 57 is coupled to the positive bus 16 through a resistor 63 and to the xenon lamp through a temperature compensating diode 65.

A keep-alive network 67 comprising a series diode and resistor is connected between the ungrounded terminal of the xenon lamp and a tap 69 on the battery 13. This keep-alive circuit serves to sustain the Xenon lamp in a slightly ionized condition between clock pulses.

In a typical battery 13, if the positive terminal is considered to be at zero volts, the voltage at the negative terminal of the battery may be -37.5 volts, and the tap might be placed at a level of -31.5 volts. A fuse 71 is inserted between the resistor 27 and the ungrounded lam-p terminal.

When the switch 15 is closed, a current is permitted to flow through the resistor 27, the fuse 71, the lamp and the resistor 55. This current vaporizes the bridge wire and blows the fuse. The fuse is selected so that it will blow approximately 15 milliseconds after the bridge wire is vaporized. During this lamp activation time, the modulation circuit is maintained in the off condition by virtue of the relatively negative voltage supplied to the emitter of the transistor 21 by virtue of the large voltage drop across the resistor 27.

After the fuse opens, the voltage drop across the re sistor 27 decreases so that the emitter of the transistor 21 returns to a voltage level near that of the bus 16. This permits the first negative pulse from the clock 17 to turn on the transistor 21. When the transistor 21 is turned on, a signal is coupled to the base of the transistor 35 so as to cut off conduction in this transistor. This feeds a signal back to the transistor 21 through the resistor 33 which tends to saturate the transistor 21.

When the transistor 35 is cut off, a signal is applied to the transistors in the Darlington pair 43 which turns on these transistors and switches on the lamp current.

The Darlington pair is preferred in the present embodiment since it provides a high gain and a relatively high input impedance. However, it will be understood that any suitable amplifying device may be used in practicing the invention.

During the time in which the transistor 21 is cut off, the capacitor 39 is charged with a polarity such that the capacitor plate connected to the transistor 35 assumes a positive polarity. The charge is limited to a value determined by the Zener diode 41 and typically is in the range of 6.8 volts.

When the transistor 21 turns on, the voltage across the capacitor 39 supplies a reverse bias to the transistor 35 and holds it in the cut off condition. This voltage is also applied to the collector terminal of the transistor 57 through the line 61. The voltage drop across the resistor 55, which is proportional to lamp current, is used to drive the transistor 57. The capacitor 39 is discharged through the control transistor 57 until the capacitor voltage is reduced to the baseto-emitter voltage of the transistor 35. When the capacitor voltage reaches this level, the transistor 35 turns on and drives the transistors 21., 45, and 47 to cut 011. The transistor 21 remains in the cut 05 condition until the next negative pulse is produced by the clock source 17. This pulse initiates another cycle of operation.

The control transistor 57 acts as a constant current source that provides a linear discharge of the capacitor 39. Since the magnitude of the collector current through the transistor 57 is proportional to the base drive, large voltages on the base cause a large collector Current and a fast discharge. Small voltages across the resistor 55 cause small collector current, a slower discharge, and thus a longer on time or duty-cycle. Since the voltage across the resistor 55 is proportional to the lamp current, the duty-cycle of the output is an inverse function. of the load current. In a typical design, an average current of 12 amperes was maintained within a 110% tolerance.

The speed-up network 29 provides rapid switching of the switching transistors. The base clearing circuit interposed between the transistor 35 and the Darlington pair assures that the transistor 45 cuts off rapidly. In a typical circuit this cut 011 function was reduced to a micro second interval. The temperature compensating diode 65 compensates for temperature variations in the base-toemitter voltage of the control transistor 57.

The diode in the keep-alive network 67 is forward biased during the of? time of the modulation cycle in which the transistor 47 is cut off. This supplies keep-alive current. The diode is back-biased during the lamp on time.

The variable duty-cycle control of FIGURE 1 maintains an approximately constant average lamp current over a wide range of lamp operating voltages and during variations of resistance in the source 13. The average power dissipated in the lamp is therefore limited without encountering the losses associated with a relatively large series resistor required in some of the prior art circuits.

The circuit of FIGURE 1 provides a pulsating light output in which the fundamental frequency of the output wave has a constant r.m.s. value. This circuit is particularly useful in conjunction with photcdetection devices that can respond to such frequencies.

The power control circuits embodying the invention, however, tend to provide alight output having a constant average value and may be preferred in applications for visual reception.

The circuit of FIGURE 2 may be used when it is desired to provide a light output which is regulated by approximating the power input to the lamp and adjusting the duty-cycle supplied to the lamp in accordance with.

this approximation.

Although the power supplied to the lamp is the product of the current through the lamp and the voltage across the lamp, the present circuit achieves an economy in circuit components by assuming that the power through the lamp is approximately proportional to the sum of the current through the lamp and the voltage across the lamp.

In the circuit or" FIGURE 2, a xenon lamp 111 is coupled to a suitable battery 113. The negative terminal of the battery is connected to the circuit by means of a negative bus 114 and a switch 115. The positive terminal of the battery is connected to the circuit through a positive bus 116. A source of negative-going clock pulses is provided by a clock 117. The clock 117 provides pulses at a rate typically in the order of 3,000 pulses per second across a resistor 11.9. These pulses are supplied to an input capacitor 121. A control transistor 123 is connected to the capacitor 121. The control transistor is further coupled to the negative bus 114 through a resistor 125. A switching transistor 127 is also coupled to receive pulses from the input capacitor 121. The transistor 12'] is coupled to the negative bus through a resistor 129.

The xenon lamp is coupled to the negative bus through a load resistor 131. The voltage across the xenon lamp is detected by means of a circuit comprising a compensating diode 133 and first and second divider resistors 135 and 137. The voltage at the tap between the divider resistors is applied to the base of the control transistor 123 r by means of a line 139.

The collector of the control transistor 123 is coupled to one terminal of the xenon lamp through a Zener diode 141 and a speed-up network 143.

The collector of the transistor 127 is coupled through a coupling diode 145 to a Darlington pair containing a first transistor 147 and a second transistor 149. The base of the transistor 147 is coupled to the base of the transistor 149 through a resistor 151 and the bases of both of these transistors are coupled to the positive bus 116 through an inductor 153.

One terminal of the xenon lamp is coupled to the battery through a keep-aline network 155 at a tap position 157. The same terminal of the xenon lamp is also coupled to the positive bus 116 through a fuse 159 and a resistor 161. The voltage appearing between the junction of the fuse and the resistor 161 is also coupled to the diode 145 through an additional diode 163.

Operation of the circuit of FIGURE 2 is initiated by closing the switch 115. The xenon lamp is energized through the resistors 131 and 161 and the fuse 159. Some of the current flowing through the fuse flows through the diode 163 and the resistor 129 and serves to maintain the transistors 147 and 149 in the cut off condition. The voltage drop across the diode145 oflsets the corresponding voltage drop across the diode 163. The bridge wire in the xenon lamp vaporizes after approximately 20 milliseconds and initiates the main discharge in the xenon lamp. Shortly after initiation of this discharge the fuse blows. This interrupts the fiow of current through the diode 163 and the resistor 129, and permits the transistors 147 and 149 to conduct.

The voltage across the resistor 131 is proportional to the lamp current. The voltage across the resistor 135 is proportional to the lamp voltage. The voltage drop across the diode 133 offsets the base-to-emitter voltage drop of the transistor 123 and provides temperature compensation to compensate for corresponding temperature changes in the transistor 123. Thus the voltage appearing on line 139 is indicative of the sum of the lamp current and the lamp voltage. The amount of collector current flowing through the transistor 123 is thus determined by the Voltage on the line 139 and the voltage drop occurring across the resistor 125. Under these conditions, substantially no current is flowing through Zener diode 141 so that the collector current from the transistor 123 functions to charge the capacitor 121.

Eventually the voltage across the capacitor 121 and the resistor 119 exceeds the base-to-ernitter voltage of the transistor 127 and this transistor begins to conduct current. This switches off the transistors 147 and 149. The resulting transient in the inductor 153 speeds up the switch off time of the transistor 149 and prevents thermal runaway.

The voltages on the collectors of the transistors 147 and 149 rise towards the negative battery voltage. This causes the Zener diode to break down and permit the trausistor 127 to conduct current. Transistor 127 is maintained in its conductive state as long as current flows through the xenon lamp. The speed-up network 143 provides a transient which accelerates switching during this operation.

During the occurrence of a negative pulse from the clock 117, the capacitor 121 charges through the base-toemitter junction of the transistor 127. At the termination of the pulse, the capacitor 121 discharges through the resistor 119 and the base-to-emitter junction of the transistor 127 until the transistor 127 is switched on. The capacitor 121 continues to discharge and effectively recharges in the reverse direction through the transistor 123 at a rate controlled by the base voltage applied to the transistor 123. Since this base voltage is indicative of the sum of the lamp voltage and current, the rate of charge of the capacitor 121 is determined by the lamp voltage and current.

When the transistor 127 is switched off, the transistors 147 and 149 are switched on and their collector voltages fall towards the voltage on the positive bus 116. This in turn, switches off the current flow through the Zener diode 141.

When the capacitor 121 charges to a suzhciently high level, the transistor 127 again begins to conduct and the cycle is repeated. Thus the on time of the circuit is determined by the lamp voltage and current.

The quiescent condition for providing a 59% duty-cycle is achieved by adjusting the value of the resistor 125. Changes in either lamp voltage or lamp current will change the duty-cycle thus varying the pulse width and controlling the average power.

The circuit of FIGURE 2 provides control signals that only approximate the power dissipated in the lamp since the power is assumed to vary as the sum of the lamp current and voltage. However, in situations in which an exact measure of power is desired, a power measuring circuit such as a Hall Effect Multiplier may be connected in the circuit of FIGURE 2 to sense the voltage drop across the load resistor 131 and the voltage drop across the xenon tube. A Hall Effect Multiplier is described in 6 US. Patent No. 2.945393 issued to Friedrich Kuhrt on July 19, 1960.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may he made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A regulator for an arc lamp comprising a source of power; means to produce a train of clock pulses; sensing means to provide a signal indicative of the instantaneous electrical input to the arm lamp to be regulated; switching means to couple said source of power to the arc lamp in response to a clock pulse; adjustable control means to actuate said switching means for a desired period of time shorter than the period of the clock pulse train; and means to adjust said control means in response to the time integral of the signal from said sensing means.

2. The regulator of claim 1 in which the electrical input to which the sensing means responds is the current through the lamp.

3. The regulator of claim 1 in which the electrical input to which the sensing means responds is the sum of the current through the lamp and the voltage across the lamp.

4. The regulator of claim 1 in which the sensing means includes a load resistor in series with the arc lamp to be controlled and the adjustable control means includes a control transistor connected to respond to the voltage drop across the load resistor.

55. The regulator of claim 6 in which the adjustable control means further contains a timing capacitor for actuating said switching means, said timing capacitor being connected to discharge through said control transistor.

6. The regulator of claim 1 in which the sensing means includes a load resistor in series with the arc lamp to be controlled and a voltage divider across the lamp and in which the adjustable control means includes a control transistor connected to respond to the sum of the voltages appearing across the load resistor and a portion of the voltage divider and a timing capacitor connected to discharge through said control transistor.

Reierenees Cited UNITED STATES PATENTS DAVID J. GALViN, Primary Examiner. 

